Grammatically Correct: The writer's essential guide to punctuation ...

Dashes
Dashes come in several sizes, so strictly speaking the term refers
to more than one mark. There is the en dash, which is roughly the
width of the capital letter N in whatever font is being used; the em
dash, which is the width of the letter M; and the 2-em dash and
3-em dash, which are the widths of two and three side-by-side Ms,
respectively. (Not every font will follow these specifications literally.
The important point is that an en dash is distinctively longer than
a hyphen, an em dash is longer than an en dash, a 2-em dash is
longer than an em dash, and a 3-em dash is the longest of all.)
The em dash is by far the most commonly used of these marks
and, except to sticklers such as editors and typesetters, is almost
always what is meant by the unqualified term dash. The en dash
has its uses but comes up in only a few specialized circumstances,
while 2- and 3-em dashes are downright esoteric.
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